KINGSTON, Jamaica (AP) — U.S. health officials on Sunday boarded a cruise ship docked in the U.S. Virgin Islands to investigate an illness outbreak that has stricken at least 300 people with gastrointestinal symptoms including vomiting and diarrhea.

The U.S. Centers for Disease Control and Prevention has said that 281, or nearly 10 percent, of the 3,050 passengers aboard Royal Caribbean International’s Explorer of the Seas have reported getting sick during a Caribbean cruise that left Cape Liberty, New Jersey, on Tuesday. Twenty-two crew members also reported feeling ill.

Janet Diaz, spokeswoman for Royal Caribbean Cruises Ltd., said CDC representatives boarded the towering, 15-deck ship as it made a port call in St. Thomas, the main island of the U.S. Virgin Islands.

At least two CDC officials, an epidemiologist and an environmental health officer, were expected to do the investigation and evaluate the outbreak response on the cruise liner. The U.S. health agency had not responded Sunday to an email and a call seeking comment about the work aboard the ship.

During the previous port call in Puerto Rico, the ship underwent “extensive and thorough sanitizing” to help prevent more people from getting sick, the company spokeswoman said. The ship bypassed a scheduled stop at the company’s fenced-in beach destination in northern Haiti to sail directly to Puerto Rico’s capital.

“This was a difficult decision to make; however, we feel it is best to make this itinerary modification to help prevent any more guests from becoming ill,” Diaz said.

The passengers and crew who fell ill have “responded well to over-the-counter medication being administered onboard the ship,” she said.

Fast-spreading norovirus is often to blame for similar symptoms sweeping closed quarters like those on cruise ships, but a determination will likely have to wait until samples are tested in a lab. Diaz said special cleaning products and disinfectants that are proven to kill norovirus are being used to clean the ship.

In a Sunday statement, Beverly Nicholson-Doty, the U.S. Virgin Islands’ tourism commissioner, said the territory was grateful for the CDC’s “quick response” and St. Thomas was ready to welcome ship passengers cleared to disembark.

On Friday, an Explorer of the Seas passenger named Arnee Dodd tweeted that she had fallen ill aboard the ship and was quarantined with the other sick people. The Connecticut woman wrote that ship employees “put a lock down on food & are constantly cleaning everything.”

It was not clear if any passengers were still being quarantined Sunday.

The ship’s next scheduled stop is the Dutch Caribbean country of St. Maarten.